You didn't clearly describe the rear location of the existing battery. My current SMB they are port and starboard back corners in fiberglass wells with covers installed by SMB. They function very well. In your case the genset may be taking up one side.... I've used 12v lead acid in both SMBs. The older, both were under a bench in boxes vented thru the floor. If you could install them in the two back corners, and had room, a very economical solution would be 2 6v golf cart batts. These I believe will fit the Gp27 box except they are about 1/2-3/4" taller. That eliminates the balancing and is, I believe, the most cost effective solution. That said, every year depending on your temperature location you are topping the water level. Good luck on your addition! AGMs are great for their lack of maintenance, but come with a cost.
Pac NW and warmer places
1995 E250 EB 5.8 2WD on to a new owner
2006 E350 EB Elect Top 2WD